Because Sony restricted its access to memory (only half of the available RAM)
This is not true. And actually Linux can use much more memory than GameOS because it is possible to use videomemory as swapfile.

the GPU
GPU is nVidia IP. Do you ever saw they disclose anything?

many other things.
Can you be more detailed?

And even average joes would have used it if it had been shipped an integrated with the system.
PS3 Linux is for CELL developers, not for average joe. And GPU (under low-end by todays standards) is totally irrelevant thing here.